The evolution of the arc tube for the high pressure sodium discharge lamp has involved numerous end closure configurations in an effort to provide a construction for carrying the discharge-sustaining electrodes, provide for the exhausting and filling of the arc tube body and an adequate seal at each end of the arc tube. Of the three, the most difficult has been providing a seal at the ends of the arc tube under the stresses produced by high temperature operation and the attendant expansion and contraction of the end closure parts during operation of the lamp. Most high pressure sodium discharge lamp failures can be attributed to a breakdown in the area of the end closure seal.
It has recently been determined that by operating the high pressure sodium discharge lamp at slightly elevated temperatures, and pressures, that a lamp having significantly improved color rendering properties can be produced. Operating at these slightly higher temperatures places an even greater burden on the end closure seal construction.
Efforts to seal off the ends of the arc tube have included constructions employing enlarged ceramic plugs as disclosed in U.S. Pat. Nos. 3,886,392 and 4,065,691 as well as the combination of ceramic plugs and ceramic end discs as disclosed in U.S. Pat. No. 4,052,635. In other constructions, improved end closures have been provided by the use of refractory metal end caps of the type disclosed in U.S. Pat. No. 4,103,200 with niobium being the preferred refractory metal for the end cap. Seals have also been developed which employ a refractory metal end closure in combination with ceramic parts as for example, those disclosed in U.S. Pat. Nos. 3,450,924; 3,832,589; and 3,825,788.
Although many of these seal constructions have proven to be, in most instances, adequate for normal operation of the high pressure sodium discharge lamp, it is believed that a construction providing a longer continuous seal path will be important in providing longevity to lamps operating at the higher temperatures required for improved color rendition.